Elastic orthodontic appliances, systems, and methods for use

ABSTRACT

An orthodontic appliance is constructed from shells shaped to receive teeth. The shells can be constructed from one or more polymer materials and stacked and also varyingly affixed to one another, which can include affixing only about the edge of each shell. The orthodontic appliance can be one of a series of similar appliances for incremental orthodontic modification of teeth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/148,078, filed Jan. 13, 2021, which is a continuation of U.S.application Ser. No. 16/775,202, filed Jan. 28, 2020, which claims thebenefit of U.S. Provisional Application No. 62/797,797, filed Jan. 28,2019. The foregoing applications are incorporated by reference herein.

FIELD OF THE INVENTION

The subject matter of the present disclosure relates generally to thefield of orthodontic devices. More particularly, the present disclosurerelates to user removable orthodontic devices.

BACKGROUND

An objective of orthodontics is to move a patient's teeth to positionswhere function and/or aesthetics are optimized. Traditionally,appliances such as braces are applied to a patient's teeth by a treatingpractitioner and the set of braces exerts continual force on the teethand gradually urges them toward their intended positions. Over time andwith a series of clinical visits and reactive adjustments to the bracesby the practitioner, the appliances to move the teeth toward their finaldestination.

More recently, alternatives to conventional orthodontic treatment withtraditional affixed appliances (e.g., braces) have become available. Forexample, systems including a series of molded plastic aligners havebecome commercially available from Align Technology, Inc., San Jose,Calif., under the trade name Invisalign® System. The Invisalign® Systemis described in numerous patents and patent applications assigned toAlign Technology, Inc. including, for example in U.S. Pat. Nos.6,450,807, and 5,975,893.

The Invisalign® System typically includes designing and fabricatingmultiple aligners to be worn by the patient before the aligners areadministered to the patient and used to reposition the teeth (e.g., atthe outset of treatment). Often, designing and planning a customizedtreatment for a patient makes use of computer-based 3-dimensionalplanning/design tools. The design of the aligners relies on computermodeling of the patient's teeth in a series of planned successive tootharrangements, and the individual aligners are designed to be worn overthe teeth, such that each aligner exerts force on the teeth andelastically repositions the teeth to each of the planned tootharrangements.

Arguably, such aligners are less noticeable than traditional bracesbecause typically aligners are constructed from a transparent material,however, many believe that aligners are easily noticeable due to theglossy sheen of the transparent material. Like traditional braces,aligners are required to be worn nearly constantly (20-22 hours a day),with breaks allowed for eating and cleaning teeth. Only small breaks areallowed because aligners do not have enough flexibility to account forteeth drifting out of alignment, which based on physical and materialcharacteristics of the aligner. Increasing the working tolerance toaccount for higher drift requires increasing the working elasticity ofan aligner, i.e., the amount an aligner can stretch to mount to teethwithout causing permanent deformation, but a highly elastic alignertypically will not provide enough force to move teeth required fororthodontic treatment. Issues like these contribute to failed results orrequire restart of treatments because patients fail to wear the alignersaccording to prescribed requirements.

SUMMARY OF THE INVENTION

Embodiments of the invention relate to orthodontic appliances, systems,and methods of use as summarized in the following paragraphs. Someembodiments relate to orthodontic appliances that maximize workingelasticity.

Some embodiments relate to an orthodontic appliance that can have shellsshaped to receive teeth. The shells can be stacked and substantiallynon-affixed to one another.

Some embodiments relate to an orthodontic appliance that can have shellsshaped to receive teeth. The shells can be stacked and varyingly affixedto one another.

Some embodiments relate to an orthodontic appliance that can have aplurality of shells shaped to receive teeth being mechanically engagedprovide stiffness for applying force to reposition teeth andsubstantially non-affixed to maximize working elasticity.

Some embodiments relate to an orthodontic appliance that can have astack of mechanically engaged shells. The stack of mechanically engagedshells can have a stiffness substantially equivalent to a single shellof the same thickness as the stack. The stack of mechanically engagedshells can have the ability to at least partially mechanically disengageto increase working elasticity.

Some embodiments relate to an orthodontic appliance that can have afirst shell having cavities shaped to receive teeth. The orthodonticappliance can include a second shell than can be shaped to receive thefirst shell. The second shell can be stacked over the first shell.Surfaces between the first shell are second shell can be mechanicallyengaged but significantly non-affixed to each other.

In some embodiments, the second shell and first shell can be affixed toone another at discrete attachment locations of the first and secondshells

In some embodiments, the first shell can have a bottom first surface fordirectly engaging the teeth and a top first surface opposite the bottomfirst surface. The second shell can have a bottom second surface formechanically engaging the top first surface of the first shell and a topsecond surface opposite the bottom second surface. The top first surfaceand the bottom second surface are not significantly affixed to eachother.

In some embodiments, the discrete attachment locations of fixationcomprise less than 1-80% of the combined surface areas of the top firstsurface and the bottom second surface.

In some embodiments, the first shell has a first edge between top firstand bottom first surfaces, and the second shell has a second edgebetween top second and bottom second surfaces, wherein the discreteattachment locations are located about the first and second edges.

In some embodiments, the shells can be made of the same material.

In some embodiments, a relatively highly elastic shell can be providedbetween said shells.

In some embodiments, the shells can include a first shell that can havecavities shaped to receive teeth, and at least one additional shell thatcan be shaped to receive the first shell, wherein the at least oneadditional shell can be stacked over the first shell.

In some embodiments, the first shell and at least one additional shellcan be affixed to one another at discrete attachment locations of thefirst and at least one additional shells.

In some embodiments, the first shell can have a bottom first surface fordirectly engaging the teeth and a top first surface opposite the bottomfirst surface. The at least one additional shell can have a bottomsecond surface for mechanically engaging the top first surface of thefirst shell and a top second surface opposite the bottom second surface.The top first surface and the bottom second surface can be arranged tonot be significantly affixed to each other.

In some embodiments, the first shell can have a first edge between topfirst and bottom first surfaces, and the at least one additional shellcan have a second edge between top second and bottom second surfaces.The discrete attachment locations can be located about the first andsecond edges.

In some embodiments, the first shell can have a bottom first surface fordirectly engaging the teeth and a top first surface opposite the bottomfirst surface and a first edge defined therebetween. The at least oneadditional shell can have a bottom second surface for mechanicallyengaging the top first surface of the first shell and a top secondsurface opposite the bottom second surface and a second edge definedtherebetween. Some or all of the first edge and the second edge can bearranged to not contact one another.

In some embodiments, the at least one additional shell can be a secondshell, and the shells can include a third shell.

In some embodiments, the shells can only consist of the first, second,and third shells.

In some embodiments, the second edge can be affixed to the top or bottomfirst surface.

In some embodiments, the first edge can be affixed to the bottom or topsecond surface.

In some embodiments, the first edge and the second edge can be separatedby 0.2-2.0 mm.

In some embodiments, the at least one additional shell can be a secondshell, and the shells can include a third shell having a bottom secondsurface for mechanically engaging the top second surface of the secondshell and a top third surface opposite the bottom third surface and athird edge defined therebetween. Some or all of the first edge, secondedge, and third edge can be arranged to not contact one another.

In some embodiments, the first edge, second edge, and third edge areseparated by 0.2-3.0 mm.

In some embodiments, all of the shells can be constructed from a sametype of material.

In some embodiments, varyingly affixed can mean having attachmentlocations that can be less than 1-80% of the combined surface areas ofthe shells.

In some embodiments, varyingly affixed can mean having attachmentlocations that can be less than 1-60% of the combined surface areas ofthe shells.

In some embodiments, varyingly affixed can mean having attachmentlocations that can be less than 1-40% of the combined surface areas ofthe shells.

In some embodiments, varyingly affixed can mean having attachmentlocations that can be less than 1-20% of the combined surface areas ofthe shells.

Some embodiments relate to a system for repositioning teeth from aninitial tooth arrangement to a final tooth arrangement. The system caninclude a plurality orthodontic appliances shaped to receive andreposition teeth. The plurality orthodontic appliances can include atleast one aspect of the orthodontic appliances described herein.

Some embodiments relate to a method for repositioning teeth from aninitial tooth arrangement to a final tooth arrangement. The method caninclude steps of incrementally using the system.

In some embodiments, the at least one orthodontic appliance of thesystem can be used less than 12 hours a day.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of at least certain embodiments, referencewill be made to the following Detailed Description, which is to be readin conjunction with the accompanying drawings.

FIG. 1 is a perspective view of a jaw and an orthodontic appliance,according to some embodiments.

FIG. 2 is an exploded view of an orthodontic appliance, according tosome embodiments.

FIG. 3A is a connection schematic for an orthodontic appliance,according to some embodiments.

FIG. 3B is a detail view of a connection schematic for an orthodonticappliance, according to some embodiments.

FIG. 4 is a perspective view of a process for molding an orthodonticappliance, according to some embodiments.

The figures depict various embodiments of the present invention forpurposes of illustration only, wherein the figures use like referencenumerals to identify like elements. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated in the figures may be employedwithout departing from the principles of the invention described herein.

DETAILED DESCRIPTION

Embodiments are disclosed that relate to orthodontic appliancesconstructed from multiple shells for the purpose of maximizing workingelasticity, which is defined herein as the capability of an orthodonticappliance to elastically deform to attach to an initial location of theteeth. This flexibility can allow an orthodontic appliance to obtain agreater range of initial tooth arranging (i.e., flexing) positions thatdiffer from the appliance's target tooth arranging (i.e., resting)position. Possible benefits include greater break time (e.g., 8-12hours) between required wear periods and greater latitude for patientnon-adherence to required wear-times, and hence increased efficacy.

Before the present invention is described in greater detail, it is to beunderstood that this invention is not limited to particular embodimentsdescribed, as such can, of course, vary. It is also to be understoodthat the terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to be limiting, sincethe scope of the present invention will be limited only by the appendedclaims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range, is encompassed within the invention. The upper and lowerlimits of these smaller ranges can independently be included in thesmaller ranges and are also encompassed within the invention, subject toany specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present invention, representativeillustrative methods and materials are now described.

It is noted that, as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise. It is further noted that the claimsmay be drafted to exclude any optional element. As such, this statementis intended to serve as antecedent basis for use of such exclusiveterminology as “solely,” “only” and the like in connection with therecitation of claim elements or use of a “negative” limitation.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which can be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentinvention. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

FIG. 1 provides an appropriate starting point in a detailed discussionof various embodiments of the present invention with respect to toothrepositioning appliances designed to apply repositioning forces toteeth. An orthodontic appliance 10 can be worn by a patient in order toachieve an incremental repositioning of individual teeth in the jaw 12.The orthodontic appliance 10 can include a shell having teeth-receivingcavities that receive and resiliently reposition the teeth. In someembodiments, a polymeric appliance can be formed from a sheet ofsuitable layers of polymeric material. An appliance can fit over allteeth present in an upper or lower jaw, or less than all of the teeth.

In some embodiments, only certain teeth received by an appliance will berepositioned by the appliance while other teeth can provide a base oranchor region for holding the appliance in place as it applies forceagainst the tooth or teeth targeted for repositioning. In some cases,many or most, and even all, of the teeth will be repositioned at somepoint during treatment. Teeth that are moved can also serve as a base oranchor for holding the appliance as it is worn by the patient.Typically, no wires or other means will be provided for holding anappliance in place over the teeth. In some cases, however, it may bedesirable or necessary to provide individual anchors on teeth withcorresponding receptacles or apertures in the appliance so that theappliance can apply a selected force on the tooth. Basic methods fordetermining an orthodontic treatment plan using a series of incrementedappliances as well as instructions for molding orthodontic appliances,are described in U.S. Pat. Nos. 6,450,807, and 5,975,893, which areincorporated by reference herein, but only to an extent that thosepatents do not contradict the newer teachings disclosed herein.

An appliance can be designed and/or provided as part of a set of aplurality of appliances. In such an embodiment, each appliance may beconfigured so a tooth-receiving cavity has a geometry corresponding toan intermediate or final tooth arrangement intended for the appliance.The patient's teeth can be progressively repositioned from an initialtooth arrangement to a target tooth arrangement by placing a series ofincremental position adjustment appliances over the patient's teeth. Atarget tooth arrangement can be a planned final tooth arrangementselected for the patient's teeth at the end of all planned orthodontictreatment. Alternatively, a target arrangement can be one of manyintermediate arrangements for the patient's teeth during the course oforthodontic treatment. As such, it is understood that a target tootharrangement can be any planned resulting arrangement for the patient'steeth that follows one or more incremental repositioning stages.Likewise, an initial tooth arrangement can be any initial arrangementfor the patient's teeth that is followed by one or more incrementalrepositioning stages.

The orthodontic appliances can be generated all at the same stage or insets or batches, e.g., at the beginning of a stage of the treatment, andthe patient wears each appliance until the pressure of each appliance onthe teeth can no longer be felt or has resulted in the maximum amount ofexpressed tooth movement for that given stage. A plurality of differentappliances (e.g., set) can be designed and even fabricated prior to thepatient wearing any appliance of the plurality. After wearing anappliance for an appropriate period of time, the patient replaces thecurrent appliance with the next appliance in the series until no moreappliances remain. The orthodontic appliances are generally not affixedto the teeth and the patient may place and replace the appliances at anytime during the procedure (e.g., patient-removable appliances).

The final orthodontic appliance or several appliances in the series mayhave a geometry or geometries selected to overcorrect the tootharrangement, i.e., have a geometry which would (if fully achieved) moveindividual teeth beyond the tooth arrangement which has been selected asthe “final.” Such over-correction may be desirable in order to offsetpotential relapse after the repositioning method has been terminated,i.e., to permit movement of individual teeth back toward theirpre-corrected positions. Over-correction may also be beneficial to speedthe rate of correction, i.e., by having an appliance with a geometrythat is positioned beyond a desired intermediate or final position, theindividual teeth will be shifted toward the position at a greater rate.In such cases, the use of an appliance can be terminated before theteeth reach the positions defined by the appliance.

FIG. 2 shows an exploded view of an example of the orthodontic appliance10. The orthodontic appliance 10 can include a first shell 14 having ateeth engaging surface and an opposite upper surface. The orthodonticappliance 10 can also include a second shell 16 having a lower-shellengaging surface and an opposite upper surface that is exposed to themouth. Optionally, one or more additional shells 18 can be locatedbetween the first shell 14 and the second shell 16. In some embodiments,the more shells that are used, the greater the working elasticity of theorthodontic appliance 10, assuming use of the same material for eachshell.

While the orthodontic appliance 10 is shown in an exploded view for thepurpose of better understanding, the shells of the orthodontic appliance10 are intended to be mechanically engaged with one another in a stack.“Mechanically engaged” is defined herein as the substantiallynon-affixed or varyingly affixed engagement between one or more shellsto approximate the strength of a single shell appliance of approximatelythe same thickness as the stacked shells. Mechanical engagement can beobtained by stacking the shells while having the lower-shell engagingsurface of the second shell largely conforming to the upper surface ofthe first shell. In some embodiments, shells can be stacked loosely,i.e., without a compressive or an interference fit between shells orsuch that an upturned stack of shells self-disassembles, before beingmade substantially non-affixed or varyingly affixed. The shells aresubstantially non-affixed (or varyingly affixed) because a substantialamount of surface areas between the shells are not bonded or otherwisemade inseparable through some process, with the remaining surfaces beingaffixed. In some embodiments, substantially non-affixed or varyinglyaffixed shells have less than 1-2%, 1-5%, 1-10%, 1-20%, 1-40%, 1-60%, or1-80% of the combined contacting surfaces of the shells affixed. Thearea of non-fixation can be limited according to the needs of theappliance, hence, in some embodiments, a majority the surface areas ofthe appliance are affixed, while the remaining part is non-affixedbecause only the latter requires high working elasticity.

In some embodiments, the lack of substantial fixation between shellsprovides greater working elasticity to the orthodontic appliance 10because the teeth-engaging shell can flex more due to being thinnerwhile the outer shells are allowed to flex in multiple directions awayfrom the teeth-engaging shell. In some embodiments, this can result inpartial mechanical disengagement between some of the engaging surfacesof the shells, however the disengagement is not enough to significantlyimpair flexural modulus of the device required for aligning the teeth tothe target position.

FIG. 3A shows a schematic for affixing the shells of the orthodonticappliance 10 at discrete locations. Each encircled “X” represents apossible point of fixation between the shells. Alternatively, as shownby the dashed line, the edges of each shell can serve as a continuous ornon-continuous area of fixation. Generally, the more fixation provided,the less working elasticity the orthodontic appliance 10 will have.Points of fixation can be determined based on the amount of workingelasticity required, which teeth are being moved, and which teeth areserving as anchors. Alternatively, the shells can be uniformly andweakly bonded with a highly elastic material of low cohesive strengththat allows for a large amount of stretching and/or shearing. Suchembodiments are substantially non-affixed or varyingly affixed becausethe working flexibility of such an orthodontic appliance are maintaineddue to the properties of the weak bond.

In some embodiments, shells of the orthodontic appliance 10 can benon-identical such that surface areas of one shell is greater or lessthan another shell. Accordingly, in some embodiments, edges, which aredefined by the top and bottom surfaces of each shell, of such shells canbe separated by gaps (e.g. 0.20-3.0 mm), as depicted by FIG. 3B, whichshows an example with three shells 14, 16, 18 and three edges 14 a, 16a, 18 a. In some embodiments, referring to the arrangement shown at FIG.2 , bottom-most shell 14 can have the greatest surface area, resultingin edge 14 a being at the bottom most position, shown, with shells 18and 16 respectively having smaller surfaces areas such that edge 16 a isat the top-most position. In such embodiments, the shells 14, 16, 18 arestacked such that steps formed by edges 14 a, 16 a, 18 a face outward,away from the teeth. In some embodiments, referring to the arrangementshown at FIG. 2 , top-most shell 16 can have the greatest surface area,resulting in edge 16 a being at the bottom most position, shown, withshells 18 and 14 respectively having smaller surfaces areas such thatedge 14 a is at the top-most position. In such embodiments, the shells14, 16, 18 are stacked such that inward facing steps formed by edges 14a, 16 a, 18 a face inward, i.e., towards the teeth.

Providing one or more of such gaps can be used to tune flexural modulusof the orthodontic appliance 10 and also result in less tongueirritation to the patient that can occur due to material thickness whereedges are bonded at the same location. To alleviate irritation, gaps canbe placed in areas that face inwards towards the mouth, resulting instepped edges (e.g., edges 14 a, 16 a, 18 a) facing the tongue, or thetooth-engaging shell can have a smaller surface area than shells stackedthereon, resulting in interior, tooth-facing steps and a single shelledge (e.g., edge 16 a) that can contact the tongue. In some embodiments,the bottom-most, tooth-engaging shell, can have a greater or lessertotal surface area than a second shell stacked thereon, which can resultin at least a portion of the edge of the second shell being separatedfrom the edge of the tooth-engaging shell. In some embodiments, onlyportions of the edges that face towards the mouth have such a gap, andin other embodiments, a uniform or non-uniform gap can exist between theentirety of edges. In some embodiments, the orthodontic appliance 10 caninclude shells, each having different surface areas.

The shells can have thicknesses ranging from 0.001-0.015 inches thick,and can be constructed from a polyester, a co-polyester, apolycarbonate, a thermoplastic polyurethane, a polypropylene, apolyethylene, a polypropylene and polyethylene copolymer, an acrylic, acyclic block copolymer, a polyetheretherketone, a polyamide, apolyethylene terephthalate, a polybutylene terephthalate, apolyetherimide, a polyethersulfone, a polytrimethylene terephthalate ora combination thereof. In some embodiments, shells are coated withlubricous materials or provided with surface treatments to decreasefriction between the shells. In some embodiments, interior portions ofthe shells are treated with hydrophobic coatings to prevent liquidintrusion into the shells. In some embodiments, shells of relativelymore flexibility can be used in conjunction with stiffer shells.Flexible shells can be constructed from hydrogels, styrenic blockcopolymers (SBC), silicone rubbers, elastomeric alloys, thermoplasticelastomers (TPE), thermoplastic vulcanizate (TPV) elastomers,polyurethane elastomers, block copolymer elastomers, polyolefin blendelastomers, thermoplastic co-polyester elastomers, thermoplasticpolyamide elastomers, or a combination thereof. Flexible shells may alsoprovide the benefit of a gasket to prevent liquid intrusion between theshells.

FIG. 4 depicts an example of a basic process 30 for forming anorthodontic appliance. As shown, a material 32 can be formed into anorthodontic appliance 36. The material 32 can be of one layer to form asingle shell or multiple non-affixed layers of material to form multipleshells at once. In this example process, the tooth positioning appliance36 can be produced with the use of a physical tooth model, or mold, 34.The tooth positioning appliance 36 can be produced by heating thethermoformable material 32 and then vacuum or pressure forming thematerial over the teeth in the physical tooth model 34. The toothpositioning appliance 36 is a direct representation of the physicaltooth model. In some embodiments, material 32 is dimensioned (e.g., 120mm and/or 125 mm diameter circle) for ready processing on a commerciallyavailable forming device (e.g., Erkoform®, Erkoform-3dmotion Biostar®,Ministar S®, Drufomat Scan®, Drufosmart®, Essix® SelectVac®). Guidelinesfor operating such forming devices can be found at Scheu DentalTechnology, Biostar Operating Manual, DE/GB/FR/IT/ES/1.000/06/19 G REFPM 0113.01; Scheu Dental Technology, Application booklet for thepressure moulding technique, GB 2.000/07/19 G REF 0111.02; Erkodent,Thermoforming, S15-3106-48; Erkodent, Erkoform 3D, 61-8002-2; Erkodent,Erkoform-3D+ Instructions, BA-Erkoform-3d+-anl-EN-04-04-2019, which areincorporated by reference herein.

After formation, shells can be affixed to one another according to thedesired working elasticity required for the patient. Methods of fixationinclude chemical bonding, localized melting, fasteners, and/or localizedphysical deformation to key the shells together. Before or afterfixation takes place, excess material from the sheet can be trimmed toform a final tooth positioning appliance that can be used fororthodontic treatment of a patient. The edges of the shells can besealed with a flexible material such as silicone to prevent liquidintrusion.

One or a series of physical tooth models, such as the model describedabove, may be used in the generation of elastic repositioning appliancesfor orthodontic treatment. Similar to the process above, each of theappliances can be generated by thermoforming a multilayer polymericmaterial over a mold of a desired tooth arrangement to form a dentalappliance. The tooth positioning appliance of the desired tootharrangement generally conforms to a patient's teeth but is slightly outof alignment with the initial tooth configuration. Placement of theelastic positioner over the teeth applies controlled forces in specificlocations to gradually move the teeth into the desired configuration.Repetition of this process with successive appliances comprising newconfigurations eventually moves the teeth through a series ofintermediate configurations to a final desired configuration.

Throughout the foregoing description, and for the purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the described techniques. It will beapparent, however, to one skilled in the art that these techniques canbe practiced without some of these specific details. Although variousembodiments that incorporate these teachings have been shown anddescribed in detail, those skilled in the art could readily devise manyother varied embodiments or mechanisms to incorporate these techniques.Also, embodiments can include various operations as set forth above,fewer operations, or more operations; or operations in an order.Accordingly, the scope and spirit of the invention should be judged interms of the claims, which follow as well as the legal equivalentsthereof.

What is claimed is:
 1. An orthodontic appliance comprising: a pluralityof shells shaped to receive teeth, wherein the plurality of shells arestacked and varyingly affixed to one another, wherein the plurality ofshells has variation in flexibility of the shells in the plurality ofshells, wherein the plurality of shells comprise: a first shell havingcavities shaped to receive teeth, wherein the first shell has: a bottomfirst surface for directly engaging the teeth; a top first surfaceopposite the bottom first surface; and a first edge defined between thebottom first surface and the top first surface; and a second shellshaped to receive the first shell, wherein the second shell is stackedover the first shell and has: a bottom second surface for mechanicallyengaging the top first surface of the first shell; a top second surfaceopposite the bottom second surface; and a second edge defined betweenthe bottom second surface and the top second surface; wherein some orall of the first edge and the second edge are in contact with oneanother and free to move relative to one another.
 2. The orthodonticappliance of claim 1, wherein the plurality of shells has variation inshell thicknesses, and wherein each of the first and second shells havethicknesses ranging from 0.001-0.015 inches thick.
 3. The orthodonticappliance of claim 1, wherein the first shell has greater flexibilitythan the second shell.
 4. The orthodontic appliance of claim 3, whereinthe first and second shells are constructed from a same type ofmaterial, and wherein the first shell is thinner than second shell. 5.The orthodontic appliance of claim 4, wherein each of the first andsecond shells have thicknesses ranging from 0.001-0.015 inches thick. 6.The orthodontic appliance of claim 4, further comprising one or moreadditional shells shaped to receive the first and second shells, whereinthe one or more additional shells are stacked over the first and secondshells.
 7. The orthodontic appliance of claim 3, wherein materials usedto construct the shells of the plurality of shells provides thevariation in flexibility of the shells in the plurality of shells. 8.The orthodontic appliance of claim 3, wherein thicknesses of the shellsof the plurality of shells provides the variation in flexibility of theshells in the plurality of shells.
 9. The orthodontic appliance of claim1, further comprising one or more additional shells shaped to receivethe first and second shells, wherein the one or more additional shellsare stacked over the first and second shells.
 10. The orthodonticappliance of claim 9, wherein the plurality of shells has variation inshell thicknesses, and wherein each of the first, second, and one ormore additional shells have thicknesses ranging from 0.001-0.015 inchesthick.
 11. The orthodontic appliance of claim 10, wherein the pluralityof shells comprises shells of relatively more flexibility used inconjunction with stiffer shells.
 12. The orthodontic appliance of claim11, wherein the shells of relatively more flexibility are constructedfrom hydrogels, styrenic block copolymers (SBC), silicone rubbers,elastomeric alloys, thermoplastic elastomers (TPE), thermoplasticvulcanizate (TPV) elastomers, polyurethane elastomers, block copolymerelastomers, polyolefin blend elastomers,
 13. The orthodontic applianceof claim 9, wherein the first shell has greater flexibility than thesecond shell and the one or more additional shells.
 14. The orthodonticappliance of claim 13, wherein the first, second, and one or moreadditional shells are constructed from a same type of material, andwherein first shell is thinner than the second shell and the one or moreadditional shells.
 15. The orthodontic appliance of claim 14, whereineach of the shells of the plurality of shells have thicknesses rangingfrom 0.001-0.015 inches thick.
 16. The orthodontic appliance of claim 9,wherein materials used to construct the shells of the plurality ofshells provides the variation in flexibility of the shells in theplurality of shells.
 17. The orthodontic appliance of claim 9, whereinthicknesses of the shells of the plurality of shells provides thevariation in flexibility of the shells in the plurality of shells. 18.The orthodontic appliance of claim 9, wherein the plurality of shellscomprises shells of relatively more flexibility used in conjunction withstiffer shells, and wherein the shells of relatively more flexibilityare constructed from hydrogels, styrenic block copolymers (SBC),silicone rubbers, elastomeric alloys, thermoplastic elastomers (TPE),thermoplastic vulcanizate (TPV) elastomers, polyurethane elastomers,block copolymer elastomers, polyolefin blend elastomers, thermoplasticco-polyester elastomers, thermoplastic polyamide elastomers, or acombination thereof.
 19. The orthodontic appliance of claim 1, whereinmaterials used to construct the shells of the plurality of shellsprovides the variation in flexibility of the shells in the plurality ofshells.
 20. The orthodontic appliance of claim 1, wherein thicknesses ofthe shells of the plurality of shells provides the variation inflexibility of the shells in the plurality of shells.